1. Technical Field
The invention relates generally to an improved suspension system for land vehicles. More particularly, the invention relates to trailing beam air suspension systems. Specifically, the invention relates to trailing beam air suspension systems having a center beam.
2. Background Information
With the advent following World War II of large load carrying capacity trucks and trailers in this country, came the need to provide vehicles with multiple axles for increasing the capacity of trucks over that of previously existing designs. While the use of additional axles effectively increased load-carrying capacity, it was soon realized that as the number of load bearing axles increased on a given vehicle, a number of difficulties arose. Specifically, tire scuffing, loss in fuel economy and the inability to safely corner, all work problems associated with multiple axle vehicles. Mitigation of these problems was a primary concern to the industry, which concern resulted in the development of a variety of suspension systems, both liftable and non-liftable. Liftable suspensions could be selectively raised from the road surface or lowered into engagement with the road surface when needed, thereby mitigating a number of the aforementioned problems. Additionally, non-liftable axles have been designed for a variety of purposes, and specifically a number of specialty chassis-cab type vehicles require additional load-carrying capacity. More specifically, auxiliary suspension systems are necessary for trash compactor trucks and concrete mixing and delivery vehicles. Cab-chassis trucks of this type require additional suspensions as the truck has a relatively large weight when compared to the overall vehicle length.
Suspension systems may take a variety of forms, including parallelogram suspensions, and leading and trailing beam-type suspensions. Generally, leading and trailing beam-type suspensions include a pair of longitudinally extending beams which may be either flexible or rigid, one of which is located adjacent each of two longitudinally extending frame rails located beneath the body of the truck or trailer. These beams are pivotally connected at one end to a hanger bracket extending downwardly from the frame, with an axle extending between the beams adjacent the other end. Additionally, an air or coil spring is generally positioned intermediate each frame rail and a corresponding beam. The beam may extend forwardly or rearwardly of the pivot, thus defining a leading or trailing beam suspension respectively.
Beam-type suspension systems are used on a significant number of trucks and trailers, and must have sufficient strength to resist lateral and axial deflection while remaining stable. Lateral forces act on a suspension system in a variety of ways with the most common being that lateral forces act on a suspension as a vehicle negotiates a turn. As the vehicle turns, shear stress acts between the tire and the road surface causing a lateral stress to be transferred through the tire-wheel assembly to the axle. The axle, being rigidly attached to the suspension, transfers the lateral force into the beam causing it to deflect laterally. This lateral deflection can be extreme, and under certain loading conditions, can cause the tires to contact the vehicle frame rails.
Roll stability refers to the counter-acting forces operating on the ends of an axle causing one end of the axle to raise relative to the frame a distance greater than the other end of the axle. Roll instability is encountered when the vehicle frame tilts or rolls relative to the axle; for example, when the vehicle negotiates a turn such that the centrifugal and acceleration forces reduce the downward forces acting on the inside wheel of the turn, and increase the downward force acting on the outside wheel of the turn. Roll instability is also realized when the axle moves relative to the frame; for example, during diagonal axle walk.
Diagonal axle walk occurs when the wheels of the opposite ends of the axle encounter unlike irregularities in a road or off-the-road surface, such as when one wheel rides over a curb. As the wheel rides over the curb, an upward force acts on that wheel, and a counteracting downward force acts on the wheel not riding over the curb. If the suspension is unable to provide flexibility between the axle and the frame as the tire-wheel assembly travels over the curb or ground irregularity, or alternatively to provide flexibility between the axle and the frame as the vehicle negotiates a turn, the suspension will be roll rigid, and may cause axle breakage or over stress vehicle components, such as the frame. As such, beam-type suspensions must be roll stable while providing sufficient vertical support to retain the vehicle above the road surface.
Further, most vehicles designed with a beam-type suspension have a path of travel which is parallel to the frame rails extending longitudinally under the vehicle. For vehicles having only a front and a rear axle, the vehicle path of travel is generally defined by the parallel and spaced apart rear tires such that the direction of travel of the rear tires defines the path of travel of the vehicle. For vehicles having only a front and a rear axle, this path of travel is adequate and safe even if the rear tires are not positioned parallel with the vehicle frame rails. However, when multiple axles are utilized, such as when auxiliary suspension systems are provided on a vehicle, the path of travel of each axle must be aligned with the line of travel of the remaining axles carried by the vehicle for safe vehicle operation.
Specifically, if one axle is aligned with the longitudinal frame rails extending under the vehicle, and a second axle is offset relative to the longitudinal frame rails of the vehicle, as the vehicle moves over the road surface, one axle and its associated tire-wheel assemblies will track along the path of travel of the vehicle, while the second axle, which includes tire-wheel assemblies which do not rotate in a direction parallel to the path of travel of the vehicle, will drag under the vehicle increasing tire scuffing, tire wear, and creating a generally unsafe condition. When multiple axles are utilized, generally all tires affect the vehicle path of travel to some degree such that if one axle is offset relative to the vehicle path of travel, all tires will scuff, and drag under the vehicle. Additionally, as the tires drag under the vehicle due to their misalignment, they continually add lateral forces to the suspension system, and consequently to the vehicle frame substantially reducing the life span of both the vehicle frame and suspension system components.
However, if the axles are aligned relative to the frame rails such that the tires rotate in a line parallel to the vehicle path of travel, the tire-wheel assemblies will rotate smoothly under the vehicle substantially increasing vehicle safety and vehicle performance as well as substantially increasing tire life.
For the above reasons, and specifically for safety and vehicle performance, it is necessary that each axle be carefully aligned with the vehicle, and with other load bearing axles carried by the vehicle to present a plurality of parallel and spaced apart tire-wheel assemblies for engaging the road surface and defining the precise direction of vehicle movement along the vehicles path of travel. Such alignment is difficult for a number of reasons. Trailers as well as suspension systems may be manufactured out of tolerance, vehicle frame rails may not be perfectly parallel, and suspension systems may not be accurately mounted to the frame rails. These problems may be especially pronounced when suspension systems are added to existing equipment which may have experienced significant use.
Thus, to accommodate for the above inconsistencies in manufacturing and suspension system installation, an alignment mechanism is often included as part of the suspension system such that after the suspension system is installed on a vehicle, the axle may be moved relative to the vehicle to assure that the tire-wheel assemblies rotatably depending from the axle are substantially parallel to the vehicle path of travel. While a significant number of devices have been provided for this purpose, axle alignment continues to be a difficult process. Specifically, adjusting the axle relative to the beams has a number of problems associated therewith. Alignment of the axle relative to the beam often includes welding the adjustment collar to the mounting bracket after initial alignment. As such, it is difficult and expensive to realign the axle after the vehicle has been in service.
An additional problem associated with trailing beam type suspensions is the increased torque load which is input into the axle. More specifically, inasmuch as the beams are spaced apart a distance from 35 inches to 41 inches, and each beam pivot point receives between 20,000 and 30,000 pounds of force when engaging in roll or diagonal axle walk, with each beam length being approximately 20 inches, it is not uncommon for the axle to be subjected to 50,000 foot pounds of torque in the area intermediate the respective leading or trailing beams. The axle is thus subjected to extremely high torque loads substantially affecting the axle and its operational characteristics. Additionally, the central portion of the axle positioned intermediate the trailing beams is not reinforced, thereby further effecting the axle resistance to torque load.
The need thus exists for a suspension system which is lightweight, is roll stable, and provides adequate vertical load-carrying characteristics, and which is resistant to lateral and longitudinal axial forces. Additionally, the need exists for a suspension system which provides an axle to beam connection which is lightweight, easy to assemble, simple to manufacture and easy to align relative to the vehicle path of travel. Still further, the need exists for a suspension system which may be utilized as a tag axle, or alternatively as an auxiliary axle beneath a usual truck or trailer. The need also exists for a suspension system which substantially eliminates axle torque while strengthening the central portion of the axle.